1. Technical Field
The present invention relates to an apparatus and method for injecting fluid into and producing fluid from a subterranean formation via a well in fluid communication with the formation, and more particularly, to an apparatus and method for injecting a gas into a subterranean formation via a well in fluid communication therewith and for producing both gas and liquid from the formation via the well.
2. Description of Related Art
Liquid hydrocarbons present in a subterranean hydrocarbon-bearing formation are conventionally produced to the surface of the earth via a well penetrating and in fluid communication with the formation. Approximately 20 to 30 percent of the volume of liquid hydrocarbons originally present within a given subterranean formation can be produced by the natural pressure of the formation, i.e., by primary production. Accordingly, secondary and tertiary recovery processes have been developed to recover additional incremental amounts of liquid hydrocarbons originally present in the subterranean formation. One such secondary and/or tertiary recovery operation is a cyclic injection/production process. This process, sometimes referred to as "huff-n-puff", involves injecting a fluid via a single well into a subterranean hydrocarbon-bearing formation so as to contact hydrocarbons in place in the near-well bore environment of the subterranean formation surrounding the well. The well may then be "shut in" for a period of time. After the shut-in period, the well is returned to production wherein an incremental volume of liquid hydrocarbons is produced from the formation to the surface via the same well. Carbon dioxide, natural gas, flue gas, and steam have been used or proposed for use in cyclic injection/production processes.
A well penetrating and in fluid communication with a subterranean hydrocarbon-bearing formation is normally provided with casing which is cemented to the walls of the well bore and is perforated to provide fluid communication with a subterranean formation of interest. A fluid is conventionally injected into the formation via such a well by first lowering a tubing string through the cased well bore to a position juxtaposed the perforated interval of the well bore. A packer which is secured to the tubing string is then expanded into contact with the casing so as to isolate the annulus defined between the casing and tubing string from the perforated interval of casing. A fluid is then injected via the tubing string, through the perforated interval of casing, and into the subterranean hydrocarbon-bearing formation. After a suitable soak period, if any, both the fluid injected into the formation and hydrocarbons originally present in the formation are produced via the tubing string to the surface. However, once the well ceases to flow from the pressure created by the injection of the treatment fluid into the subterranean formation, an artificial lift must be installed within the well to produce fluid therefrom. Thus, a kill fluid, for example, brine or produced water, is introduced into the annulus defined between the casing and the tubing string as the packer on the tubing string is deflated. The kill fluid balances the hydrostatic formation pressure and thereby prevents fluid from the subterranean hydrocarbon-bearing formation from entering the well and flowing to the surface. The tubing string is then removed from the well and a different tubing string which is configured to permit fluids to be produced from the well by means of a reciprocating rod pump is positioned within the well. This production tubing string may not include an expandable packer for isolating the casing/tubing annulus. A conventional reciprocating rod pump which is utilized in a well normally contains two valves. One valve is termed the traveling valve and is attached to the pump plunger while the other valve is a standing valve and is positioned within the tubing. The action of the plunger going up causes a reduction in pressure in the pump chamber which permits the standing valve to open.
One problem associated with the conventional technique for injecting and producing fluids from one well is that kill fluid invades the treated area of the subterranean formation and/or contaminates the treatment fluid previously injected into the subterranean formation thereby reducing the effectiveness of the treatment. In wells that produce fluid from a subterranean formation which contains a significant quantity of entrained or dissolved gas, the gas, including the treatment gas and/or gas, such as hydrogen sulfide or methane, originally present in the formation, will break out of solution as the pump plunger is lifted upwardly. Once out of solution, this gas will compress thereby preventing the traveling valve from opening. Thus, the pump becomes inoperable due to gas locking or gas interference. A procedure termed "tagging bottom" has been employed to unseat a traveling valve in a gas locked pump. In accordance with this procedure, the sucker rod string is lowered within a well a sufficient distance to allow the pump to physically contact the bottom of the tubing string with sufficient force to unseat the traveling valve. Thereafter the rod string is raised to reposition the pump adjacent the perforated interval.
Another problem in utilizing a reciprocating rod pump to produce fluids from a cased well bore occurs when a packer is employed to isolate the annulus between the casing and tubing. When entrained or dissolved gas is produced via perforations into a cased well bore, the pressure drop which occurs as the fluid is produced into the well bore permits the gas to break out of the produced fluid and collect under a packer. As sufficient gas pressure builds up underneath the packer, the fluid level will be depressed to a level below the pump intake thereby preventing fluid from being produced from the well bore. Thus, a need exists for an apparatus which can be used to inject fluid into and produce fluid from a subterranean hydrocarbon-bearing formation without the use of a kill fluid and without the problems associated with gas breaking out of produced fluid.
Accordingly, it is an object of the present invention to provide an apparatus for both injecting fluid into and producing fluid from a subterranean formation.
It is another object to provide a method and apparatus for injecting fluid into producing fluid from a subterranean formation which eliminates the use of a kill fluid.
It is a further object of the present invention to provide an apparatus and method for producing fluids including entrained or dissolved gas from a subterranean formation which substantially eliminates the possibility of reciprocating pump malfunction due to gas locking or interference.
It is a still further object of the present invention to provide an apparatus and method for producing fluids including entrained or dissolved gas from a subterranean formation which substantially eliminates fluid production problems associated with gas breaking out of fluid produced from the formation.
It is also an object of the present invention to provide an apparatus for injecting fluid into and producing fluid from a subterranean formation which is relatively simple in construction and can be readily assembled and disassembled at the well site to facilitate use and maintenance thereof.